Handle the competition being over

## Getting Started page

The countdown disappears and a link to the past leaderboard takes its place

## Leaderboard

If the *current* competition has ended, it uses the end of the competition as the reference date, instead of the current date. This shows the stats as of the last day of the competition.

## Dashboard

The team participation percent shows 0% because it's using the old calculations model that I didn't update. The goal was to eventually remove that whole thing, but it's more effort to do than I can put in right now *(I tried it and multiple tests failed so I stopped)*.

## Staging

This branch has been pushed to [staging](http://testcommuterchallenge.bicycleanchorage.org/)